The present invention generally relates to lightvalves and, more particularly, to improved reflective twisted nematic liquid crystal (LC) lightvalves and systems employing same.
Reflective lightvalves are becoming widely used in projection displays. Such lightvalves can be decreased in size without incurring a penalty in pixel aperture, allowing a corresponding shrinkage in size and cost of the entire projection system. Reflective lightvalves based on twisted nematic liquid crystal (TNLC) layers, such as the 45xc2x0 (degree) twist or 54xc2x0 twist modes, make use of well developed LC technology, and with relatively modest driving voltages can provide a reasonably satisfactory optical response when reproducing black, white or intermediate grayshaded image regions. A TNLC layer of twist angle xcex1 and birefringence xcex94n which is set to a thickness d satisfying:
d=(xcex/xcex94n){square root over (m2xe2x88x92(xcex1/xcfx80)2)}xe2x80x83xe2x80x83(1)
will provide reasonably high contrast within a band of wavelengths centered at xcex; in most cases this band is wide enough to project a single primary color (i.e., red, green, or blue), allowing the full image to be projected from three lightvalves. Minimum suitable thickness is obtained by setting m=1 in equation (1); the choice m=1 thus minimizes the drive voltage that must be supplied when projecting image regions of maximum brightness.
Though the intrinsic contrast of the TNLC lightvalve is in itself usually acceptable over a single color band, the contrast of the full projection system is almost always poorer than that of the lightvalve alone, due to an interaction between the TNLC lightvalve and the projection optics, discussed further below. In demanding applications, even the intrinsic contrast of the TNLC lightvalve in isolation may be marginal; however there is a known method for improving the intrinsic crossed polarizer transmission of the TNLC layer. U.S. Pat. No. 4,408,839, issued to Wiener-Avnear and entitled xe2x80x9cTwisted Nematic Liquid Crystal Light Valve with Birefringence Compensation,xe2x80x9d the disclosure of which is incorporated by reference herein, discloses a TNLC corrector layer which the illumination traverses before reaching the TNLC lightvalve; this corrector compensates the black state of the lightvalve. The twist within the TNLC corrector is chosen in such a way that the LC molecules along the exit surface of the corrector are perpendicular to the LC molecules at the adjoining entrance surface of the lightvalve. These two monomolecular sublayers then have parallel birefringences of opposite sign, and so cancel. The two TN layers are given opposite twists of equal magnitude, which means that if one considers successive additional pairs of sublayers (one from each LC layer, selecting the two sublayers to be at equal distances from the adjoining exit and entrance faces), the two sublayers in successive pairs continue to cancel each other in the above fashion (if dxcex94N for the two layers is the same). The Wiener-Avnear patent discloses that the intrinsic TNLC black state intensity is thus made zero at all wavelengths. Other two layer transmission lightvalves have been disclosed in which the dxcex94N product or twist angle is not the same in the two layers.
However, while the imperfect intrinsic black state of the TNLC lightvalve is correctable by the known double layer techniques, the prior art reflective lightvalve exhibits a number of other limitations. Examples of these limitations will be described below in the subsections (1) through (8).
(1) The spectral width of the high contrast zone is not adequate to project all three color bands from a single lightvalve, hence three lightvalves are required. While use of three lightvalves has the advantage of maximizing image brightness, there are applications where adequate brightness could be achieved at lower cost from a single lightvalve (or two lightvalves), if a single lightvalve were capable of projecting all three colors.
(2) A particular method for projecting multiple colors from a single lightvalve is to sequentially project each color component at a sufficiently high rate that the eye perceives the three components to be simultaneous. However, it is difficult to switch TNLC lightvalves rapidly enough to achieve a flicker free image in sequential mode. Switching time is approximately quadratic in the cell gap d. The equation (1) contrast condition sets a minimum attainable thickness d for the TNLC lightvalve, and hence a minimum switching time. Once an LC is chosen with the largest possible xcex94N, and the twist angle a is set to a sufficiently large value that high bright state reflectivity is obtained together with an adequately broad band of high contrast, the cell gap d of the TNLC lightvalve is then fixed by equation (1).
(3) Different cell gaps must be assigned to the red, green, and blue lightvalves, as per equation (1). Since switching speed is then different in the three color channels, the projected image of a moving object of mixed color (e.g., white) will exhibit different colors in its leading and lagging edges.
(4) These constraints on voltage and cell gap indicate that an LC with the largest possible birefringence xcex94N should be chosen. This may rule out the use of LCs with other desirable properties, such as fluorinated LCs, which have low sensitivity to UV (ultraviolet) light.
(5) Small errors in cell gap thickness d can degrade the quality of the black state. The optical effect is governed by changes in the dimensionless parameter xcex2 defined as:                     β        ≡                  π          ⁢                                                    d                ⁢                                  xe2x80x83                                ⁢                Δ                ⁢                                  xe2x80x83                                ⁢                n                            λ                        .                                              (        2        )            
In principle, a small error in cell gap can be compensated by rotating the lightvalve slightly. The orientation of minimum black state reflectivity is given by:                               Θ          xe2x80x2                =                  a          ⁢                                    tan              ⁢                              xe2x80x83                            ⁢              γ                        γ                                              (        3        )            
where
xcexxe2x89xa1{square root over (xcex12+xcex22)}.xe2x80x83xe2x80x83(4)
Unfortunately, errors in the cell gap are often the result of quasi-random process variations, and cannot be determined until the lightvalve is actually fabricated. At that point, it is no longer possible to rotate the lightvalve because it must remain aligned with the desired image orientation and with the lightvalves of the other two color channels.
(6) Even in the absence of cell gap errors, the TNLC lightvalve does not provide ideal contrast. Most commonly, these LVs (lightvalves) are used in projection systems where the LV is illuminated through a polarizing beamsplitter (PBS) and quarterwaveplate (QWP); in such systems the intrinsic black level is given by:                                           B            System                    =                                                    (                                                      2                    ⁢                                          xe2x80x83                                        ⁢                    a                    ⁢                                          xe2x80x83                                        ⁢                    β                    ⁢                                          xe2x80x83                                        ⁢                                          sin                      2                                        ⁢                    γ                                                        γ                    2                                                  )                            2                        +                                                            NA                  2                                                  n                  2                                            ⁢                                                (                                      β                    ⁢                                                                  sin                        ⁢                                                  xe2x80x83                                                ⁢                        2                        ⁢                                                  xe2x80x83                                                ⁢                        γ                                            γ                                                        )                                2                                                    ,                            (        5        )            
where NA is the numerical aperture of the projection optics (at the lightvalve), and n is the PBS substrate refractive index. The cell gap d is usually chosen to satisfy equation (1) at a single (usually central) wavelength in the color band. At this central wavelength, both terms in equation (5) are zero, but at other wavelengths, both are non-zero, and projected black regions of the image are not completely dark. When displaying darker shades of a single color (i.e., shades where one color is set to a reflectivity slightly above black state while the other two are set nominally at zero), the residual reflectivity of the two black state lightvalves causes the image color in the driven channel to be significantly desaturated, i.e., to be washed out.
As discussed above, the first term in equation (5) is intrinsic to the TNLC lightvalve itself. The second term arises from a birefringence-like effect in the TNLC layer. The QWP phase shift precisely eliminates compound angle depolarization at the single wavelength satisfying equation (1) above, but at other wavelengths the TNLC layer introduces its own phase shift which is not matched by the QWP. This phase offset is linear in small wavelength departures from the equation (1) condition. The retardance of the corrective waveplate could be set to a value other than 90xc2x0, but it is the strong dispersion in the offset phase that makes compound angle correction imperfect. The resulting black state intensity is quadratic in wavelength departure from the equation (1) centerpoint, and is also quadratic in the optical system NA. There is a trend in the projection display industry towards larger NAs to increase the light collection power of projection optics; unfortunately the equation (5) background effect conflicts with this goal.
(7) a. The e-field polarization at the lightvalve backplane is approximately linear when the reflective TN (twisted nematic) lightvalve is in black state, but the polarization is rotated relative to the input direction. The rotation angle is roughly equal to the twist angle a, which might for example be 45xc2x0. The topography of the backplane typically comprises vertical and horizontal mirror edges that follow the row, column layout of pixels in the projected image. In order to avoid depolarization from scattering off these topographical edges, it would be preferable that the polarization at the backplane be horizontal or vertical, instead of, e.g., 45xc2x0.
b. When the wavelength is offset slightly from the center wavelength, the polarization at the mirror backplane remains linear to first order, but it is rotated out of alignment with the LC molecules at the backplane, i.e., at offcenter wavelengths the polarization is rotated slightly away from the, e.g., 45xc2x0 orientation of the LC molecules. Ideally, the backplane would reflect the light back into the LC in this same slightly rotated linear polarized orientation. Such a dispersive rotation of polarization would then cancel out in the return pass through the LC. However, if there is a phase shift upon reflection from the backplane that is dependent on the incident index, such a rotation of the transmitted e-field relative to the birefringent LC can cause the light to be reflected in an elliptical polarization (due to the anisotropic LC index), and such ellipticity will not cancel in the return pass through the LC.
c. The most common manufacturing process for establishing the orientation of the LC molecules at the backplane is through rubbing of an alignment layer. This rubbing process creates artifacts when the lightvalve cell gap is maintained by spacer posts placed in the boundaries between mirror pixels. The principle advantage of spacer post technology is that it provides very accurate control of the cell gap; however a disadvantage is that spacer posts perturb the alignment of nearby LC. Incident light whose polarization is altered by that portion of the disturbed LC which is immediately adjacent to the posts will be absorbed by the low reflectivity layer which separates the pixel mirrors; thus in regions very close to the posts, the disturbed LC has little effect on the displayed image. Unfortunately, the region of disturbed LC may be considerably extended (xcx9c10 microns or m) in the direction of alignment layer rubbing. In the known reflective TN lightvalves, this rubbing direction is at an angle such as 45xc2x0 to the dark interpixel boundaries, creating visible LC disturbance in the regions over the mirrors.
Thus, for at least the above three reasons, it would be preferable to have the e-field at the backplane be aligned with both the LC molecules and the edges of the pixel topography, even at wavelengths that depart from the center wavelength defined by equation (1).
(8) To display grey image shades, a moderate voltage is applied to the TN lightvalve. Roughly speaking, the effect of applied voltage is to reduce xcex94N by increasing the tilt of the LC molecules against the substrate surfaces. The cell gap d is set such that when V=0, equation (1) is satisfied at some wavelength in the center of the color band, i.e., when V=0, the LV gives the blackest response at band center. However, after xcex94N has been reduced by a small applied voltage, equation (1) will be satisfied at a shorter wavelength than in the V=0 black state. At low voltages, the LV thus switches on first at longer wavelengths in the color band. For qualitative purposes, we can estimate the response using the first term of equation (5), which predicts that the reflected intensity increases from a level near zero to an extremum near unity as xcex2 is driven from an initial value of about 3 down to a value of about 1.1. This is illustrated in FIGS. 1A (linear scale) and 1B (log scale) for the case where xcex1=45xc2x0. Since xcex2 is inversely proportional to xcex, the longer wavelengths in the band reach the bright state extremum first. After longer wavelengths reach this extremum, further voltage increases cause the relative shortwave content of the reflected spectrum to be restored (as shorter wavelengths in the band are also driven to maximum), until the overall reflectance maximum is reached. This is illustrated in FIG. 2.
The combined effect of the non-uniform spectral response in the black state (subsection (6) above) and at intermediate shades of gray (subsection (8) above) causes the color content of most image regions to be distorted. Image chromaticity at an intermediate gray shade of a single color can differ appreciably from that of the full intensity primary. FIG. 3 plots the color coordinates of red, green, and blue image regions over a continuous range of intensities.
FIGS. 4A and 4B are diagrams illustrating the structure of a prior art lightvalve. As shown in FIG. 4A, the liquid crystal (LC) molecules comprising layer 4 are oriented by alignment layers 5 and 7. Topglass 1 and pixelated substrate 6 hold LC layer 4 in place. The cutaway, shown in FIG. 4B, shows the structure with topglass 1 removed for clarity. Spacers 11 maintain the thickness of LC layer 4. The LC molecules at the topglass-side of layer 4 (indicated schematically by arrow 2) are oriented parallel to the edges of mirror pixels 10. Arrow 3 indicates schematically that the LC molecules at the backplane are oriented at an angle such as 45 degrees relative to the edges of pixels 10. The arrows represent the projection of the LC molecules onto planes parallel to the substrate, whereas the molecules may actually exhibit a small pre-tilt against the substrate. It should be noted that FIG. 4B is not drawn to scale. Sixteen pixels are shown; each is typically 10 xcexcm (micrometers or microns) to 20 xcexcm in size. The complete light valve contains approximately 106 pixels. Typical LC thickness is about 3 xcexcm, and topglass thickness is about 1 mm (millimeter).
Despite the limitations described above, the prior art reflective TN lightvalve achieves reasonable performance using very reliable TNLC technology; technology which has been refined over a period of decades. What is needed is a reflective lightvalve that makes use of twisted nematic liquid crystal technology without suffering the above limitations.
The present invention provides an improved liquid crystal (LC) structure for lightvalves that makes use of twisted nematic liquid crystal technology without suffering the above and other limitations. In one aspect of the invention, a reflective liquid crystal lightvalve for modulating the polarization of incident light within a specified band of wavelengths into on and off states, comprises: (i) a pixelated reflective backplane; (ii) a first liquid crystal layer, positioned proximate the pixelated reflective backplane, the first liquid crystal layer being tuned in the off state to switch incident light at the center of the specified band of wavelengths into a state that is not fully off; and (iii) a second liquid crystal layer, positioned proximate the first liquid crystal layer wherein the first liquid crystal layer is positioned between the second liquid crystal layer and the pixelated reflective backplane, the second liquid crystal layer having a birefringence which, at a given depth within its thickness, is substantially equal and opposite to a birefringence of a layer within the first liquid crystal layer that is located at a matching distance from a midplane separating the first and second liquid crystal layers. Preferably, the first liquid crystal layer is a twisted nematic layer that is tuned in the off state to fully switch off light of a wavelength that is shorter than the central wavelength of the specified band of wavelengths. Further, preferably, the orientation of liquid crystal molecules in a plane at a given depth of the second liquid crystal layer, when projected onto said plane, is substantially perpendicular to the projected orientation of liquid crystal molecules of the first liquid crystal layer in a plane that is located at a matching distance from a midplane separating the first and second liquid crystal layers.
In another aspect of the invention, an LC structure comprises two LC layers, in which each LC layer is band-shifted to a dxcex94N value producing maximum contrast in the layer at a wavelength shorter than the central operating wavelength for the combined structure, wherein one of the LC layers is undriven and can be rotated relative to the driven layer.
In yet another aspect of the invention, a reflective LC lightvalve comprises two twisted nematic LC layers and a quarterwave retarder, each twisted nematic layer being set to a thickness that provides, from the individual layer, an optimum projected black state at shorter wavelengths than the wavelength band of interest, thus providing, from the combined tri-layer structure, a spectral response that, within the wavelength band of interest, is improved at all driving voltages.
Various novel optical systems utilizing one or more of the inventive lightvalve structures are also described in detail herein. However, given the inventive teachings, one of ordinary skill in the art will realize other applications and systems.
These and other objects, features and advantages of the present invention will become apparent from the following detailed description of illustrative embodiments thereof, which is to be read in connection with the accompanying drawings.